Disorders of the bone, ranging from osteoporosis to fractures, represent a set of pathological states for which there are few effective pharmaceutical agents. Treatment instead focuses on physical and behavioral interventions, including immobilization, exercise and changes in diet. It would be beneficial to have therapeutic agents that promote bone growth and increase bone density for the purpose of treating a variety of bone disorders.
Bone growth and mineralization are dependent on the activities of two cell types, osteoclasts and osteoblasts, although chondrocytes and cells of the vasculature also participate in critical aspects of these processes. Developmentally, bone formation occurs through two mechanisms, endochondral ossification and intramembranous ossification, with the former responsible for longitudinal bone formation and the later responsible for the formation of topologically flat bones, such as the bones of the skull. Endochondral ossification requires the sequential formation and degradation of cartilaginous structures in the growth plates that serve as templates for the formation of osteoblasts, osteoclasts, the vasculature and subsequent mineralization. During intramembranous ossification, bone is formed directly in the connective tissues. Both processes require the infiltration of osteoblasts and subsequent matrix deposition.
Fractures and other structural disruptions of bone are healed through a process that, at least superficially, resembles the sequence of developmental events of osteogenesis, including the formation of cartilaginous tissue and subsequent mineralization. The process of fracture healing can occur in two ways. Direct or primary bone healing occurs without callus formation. Indirect or secondary bone healing occurs with a callus precursor stage. Primary healing of fractures involves the reformation of mechanical continuity across a closely-set disruption. Under suitable conditions, bone-resorbing cells surrounding the disruption show a tunneling resorptive response and establish pathways for the penetration of blood vessels and subsequent healing. Secondary healing of bones follows a process of inflammation, soft callus formation, callus mineralisation and callus remodelling. In the inflammation stage, haematoma and haemorrhage formation results from the disruption of periosteal and endosteal blood vessels at the site of injury. Inflammatory cells invade the area. In soft callus formation stage, the cells produce new vessels, fibroblasts, intracellular material and supporting cells, forming granulation tissue in the space between the fracture fragments. Clinical union across the disruption is established by fibrous or cartilaginous tissue (soft callus). Osteoblasts are formed and mediate the mineralization of soft callus, which is then replaced by lamellar bone and subjected to the normal remodeling processes.
In addition to fractures and other physical disruptions of bone structure, loss of bone mineral content and bone mass can be caused by a wide variety of conditions and may result in significant medical problems. Changes to bone mass occur in a relatively predictable way over the life of an individual. Up to about age 30, bones of both men and women grow to maximal mass through linear growth of the endochondral growth plates and radial growth. After about age 30 (for trabecular bone, e.g., flat bones such as the vertebrae and pelvis) and age 40 (for cortical bone, e.g., long bones found in the limbs), slow bone loss occurs in both men and women. In women, a final phase of substantial bone loss also occurs, probably due to postmenopausal estrogen deficiencies. During this phase, women may lose an additional 10% of bone mass from the cortical bone and 25% from the trabecular compartment. Whether progressive bone loss results in a pathological condition such as osteoporosis depends largely on the initial bone mass of the individual and whether there are exacerbating conditions.
Bone loss is sometimes characterized as an imbalance in the normal bone remodeling process. Healthy bone is constantly subject to remodeling. Remodeling begins with resorption of bone by osteoclasts. The resorbed bone is then replaced by new bone tissue, which is characterized by collagen formation by osteoblasts, and subsequent calcification. In healthy individuals the rates of resorption and formation are balanced. Osteoporosis is a chronic, progressive condition, marked by a shift towards resorption, resulting in an overall decrease in bone mass and bone mineralization. Osteoporosis in humans is preceded by clinical osteopenia (bone mineral density that is greater than one standard deviation but less than 2.5 standard deviations below the mean value for young adult bone). Worldwide, approximately 75 million people are at risk for osteoporosis.
Thus, methods for controlling the balance between osteoclast and osteoblast activity can be useful for promoting the healing of fractures and other damage to bone as well as the treatment of disorders, such as osteoporosis, associated with loss of bone mass and bone mineralization.
With respect to osteoporosis, estrogen, calcitonin, osteocalcin with vitamin K, or high doses of dietary calcium are all used as therapeutic interventions. Other therapeutic approaches to osteoporosis include bisphosphonates, parathyroid hormone, calcimimetics, statins, anabolic steroids, lanthanum and strontium salts, and sodium fluoride. Such therapeutics, however, are often associated with undesirable side effects.
Thus, it is an object of the present disclosure to provide compositions and methods for promoting bone growth and mineralization.